Degradation of chitosan by sonication in very-low-concentration acetic acid

Chitosan is a linear copolymer composed of (1 → 4)–linked 2-acetamido-2-deoxy-β-D-glucan (GlcNAc) and 2-amino-2-deoxy-β-D-glucan (GlcN) units in varying proportions, having a high molecular weight and strong intra- and intermolecular hydrogen bondings. Sonication has become an alternative for degrading chitosan into low-molecular-weight chitosan (LMWC), chitosan oligomers and glucosamine. In this study, chitosan was treated with sonication at 40 °C and 60 °C for 30 min and 120 min with various acetic acid concentrations (0.2% v/v–1% v/v); the very-low-concentration acid solution functioned both as a solvent and catalyst. After sonication, the samples were tested for changes in molecular weight, water soluble proportion of chitosan (chitosan oligomers and glucosamine), degree of deacetylation, degree of crystallinity, and morphology. The soluble and insoluble product yields at low concentration (0.5% v/v) at 40 and 60 °C were 33.66–39.37 % and 32.43–34.26%, respectively. The main product was 5-hydroxy methyl furfural with composition 92.16–99.43%. At high concentrations (1% v/v), the soluble product and insoluble yields were 43.72–49.74% and 43.1–50.26%, respectively. The main product was glucosamine with composition 77.75–93.16% of glucosamine. There were changes in the morphology and crystallinity of the degraded chitosan, but no change in the chemical structure. The crystallinity had a tendency to increase. The degree of deacetylation tended to decrease due to the glucosamine breakage.